Interpretive Summary: Although the adverse health effects of heavy metals have been known for a long time, exposure to heavy metals continues and is increasing, due to their continued production and emission into the environment from mining, smelting, electroplating, and the use of products such as fertilizers, batteries, pigments, and plastics. Heavy metals enter the food chain via uptake into plants and heavy metals such as copper (Cu), which is the focus of this study, cause cellular toxicity from the displacement of naturally occurring co-factors from their cellular binding sites, resulting in damaged proteins and enzymes, and promotion of the formation of toxic reactive oxygen species. Also, some heavy metals such as Cu are also essential micronutrients for plants, animals and humans. Thus, for these essential micronutrients/heavy metals, understanding the cellular mechanisms of transport, accumulation and detoxification is critical for understanding micronutrient nutrition as well as the cure and prevention of heavy metal-caused diseases and for developing effective strategies for remediation of contaminated environments using plants. In this study, the plant model species, Arabidopsis thaliana, was used to study the accumulation and detoxification of Cu under conditions of Cu limitation as well as Cu excess. Novel findings were presented identifying a new transporter that is a key player in the plant transport of Cu in the plant shoot. These new findings identify a novel target for genetic manipulation with regards to both preventing heavy metals from entering the food chain via transport to the edible portions of food crops, and also helping crop plants deal with insufficient levels of essential micronutrients such as Cu in marginal soils.

Technical Abstract:
Among the mechanisms controlling copper homeostasis in plants is the regulation of its uptake and tissue partitioning. Here we characterized a newly identified member of the conserved CTR/COPT family of copper transporters in Arabidopsis thaliana, COPT6. We showed that COPT6 resides at the plasma membrane and mediates copper uptake when expressed in the Saccharomyces cerevisiae copper uptake mutant. Although the primary sequence of COPT6 contains the family conserved domains, including methionine-rich motifs in the extracellular amino terminal domain and a second transmembrane helix (TM2), it is different from the founding family member, S. cerevisiae Ctr1p. This conclusion was based on the finding that although the positionally conserved Met106 residue in the TM2 of COPT6 is functionally essential, the conserved Met27 in the amino terminal domain is not. Structure-function studies revealed that the amino terminal domain is dispensable for COPT6 function in copper replete conditions but is important under copper limiting conditions. In addition, unlike Ctr1p that interacts with itself but not with other Ctr proteins, COPT6 interacts not only with itself but also with its homolog, COPT1. Analyses of the expression pattern showed that while COPT6 is expressed in different plant organs and cell types, the bulk of its expression is located in the vasculature. We also show that COPT6 expression is regulated by copper availability that, in part, is controlled by a master regulator of copper homeostasis, SPL7. Finally, studies using the A. thaliana copt6-1 mutant and plants overexpressing COPT6 revealed its essential role during copper limitation and excess.